Autodeposition has been in commercial use on steel for about thirty years and is now well established for that use. For details, see for example, U.S. Pat. Nos. 3,063,877; 3,585,084; 3,592,699; 3,674,567; 3,791,431; 3,795,546; 4,030,945; 4,108,817; 4,178,400; 4,186,226; 4,242,379; 4,234,704; 4,636,264; 4,636,265; 4,800,106; and 5,342,694. Epoxy resin-based autodeposition coating systems are described in U.S. Pat. No. 4,180,603 (Howell. Jr.); U.S. Pat. No. 4,289,826 (Howell Jr.); U.S. Pat. No. 5,500,460 (Ahmed et al.); and International Publication Number WO 00/71337. Epoxy-acrylic blend resins for autodeposition have also been developed as described in U.S. Pat. No. 7,138,444, but these coatings provide a low gloss, semi-matte finish. The disclosures of all these patents and published patent applications are hereby incorporated by reference to the extent that they are not specifically contradicted by the below teachings.
Autodeposition compositions are usually in the form of a liquid, usually aqueous solutions, emulsions or dispersions in which active metal surfaces of inserted objects are coated with an adherent resin or polymer film that increases in thickness the longer the metal remains in the bath, even though the liquid is stable for a long time against spontaneous precipitation or flocculation of any resin or polymer, in the absence of contact with the active metal. When used in the autodeposition process, the composition when cured forms a polymeric coating. “Active metal” is defined as metal that spontaneously begins to dissolve at a substantial rate when introduced into the liquid solution or dispersion. Such compositions, and processes of forming a coating on a metal surface using such compositions, are commonly denoted in the art, and in this specification, as “autodeposition” or “autodepositing” compositions, dispersions, emulsions, suspensions, baths, solutions, processes, methods or a like term. Autodeposition is often contrasted with electrodeposition. Although each can produce adherent films with similar performance characteristics, the dispersions from which they are produced and the mechanism by which they deposit are distinctly different. Electrodeposition requires that metal or other objects to be coated be connected to a source of direct current electricity for coating to occur. No such external electric current is used in autodeposition.
Typical autodeposition coatings currently commercially available, including epoxy-acrylic based systems that provide good corrosion resistance, have a low gloss finish and may include surface inconsistencies. In many industries, the low gloss finish of the autodeposition primer was perfectly satisfactory for the application. For example, conventional autodeposition coatings have been used as primers to be subsequently coated with paint in order to achieve a glossy, “show surface” appearance. Such autodeposition primer compositions have been utilized in numerous technical applications, such as automotive components, agricultural machinery, appliances, too numerous to mention here, that require anti-corrosion and proper adhesion properties for subsequent coatings.
In certain applications, the autodeposition coating has been used without the need for topcoating. Generally, due to the low gloss nature of commercially available autodeposition coatings, this use has been limited to non-show surfaces, for example in shock absorbers, automotive seating tracks and the like. Although autodeposition coatings have been utilized for decades in these coating instances, a smooth, glossy, improved appearance with good corrosion and adhesion characteristics was not readily forthcoming. There are incidences where an improved appearance of the autodeposition coating is desired without the need for subsequent coating operations, and this has been an unmet need in the industry.
In the prior art, certain epoxy or epoxy-acrylic resins used for autodeposition applications were generally made by preparing solutions of epoxy resin and acrylic monomer in organic solvent(s). The resulting mixture was dispersed in water with surfactant, mechanically dispersed to a desired particle size and then polymerized. Because a high concentration of solvent is undesirable in an autodepositing coating bath, these solvent-rich emulsions required removal of solvent by distillation techniques known in the art. The resulting polymerized resin could then be mixed with other components and used as a component of an autodeposition coating bath. The bath forms a polymeric coating when autodeposited on a metal surface and cured. Low solvent/no solvent epoxy-acrylic autodeposition coating compositions have been developed which eliminate and/or minimize the need for process solvents, which had caused increased manufacturing costs. However, the appearance of these epoxy-acrylic resin based coatings had some shortcomings such as high surface roughness and low gloss.
As discussed above, previous autodeposition formulations have been limited to applications where a semi-matte, low gloss finish was acceptable. Prior attempts to achieve a glossy, improved appearance have not met with a high degree of success, particularly with epoxy-acrylic compositions. Direct-to-metal or corrosion protection thermoset primers, such as 2-components urethane, 2-components epoxy, and electrodeposited epoxies, are single phase low molecular weight polymeric systems with glossy, smooth finishes due to their low viscosity, but these systems lack edge corrosion performance. While traditional methods of enhancing appearance characteristics of a coating involved the use of flow additives, co-solvents, or plasticizers, among others, those solutions may impact coating volatile organic compound (VOC) content and environmental sustainability.
Previously, the characteristics of the epoxy and acrylic phases of the resin were incompatible, and had contributed to low gloss, matte and textured finish coatings. The present invention seeks to make compatible both phases and in doing so enhances the aesthetic and overall appearance properties of resulting coatings, by increasing gloss and reducing roughness, without loss of corrosion performance and without loss of adhesion to the metal substrate and any later applied coatings. Another aspect of the present invention is that the smoother, glossier coating of the invention may much easier to clean, and can be easily rinsed off by natural conditions on the roadway, thereby possibly lessening corrosion.
Up to now, autodeposition coatings have not been used successfully for automotive, “show” surfaces due to the lack of gloss and smoothness sufficient to meet manufacturer requirements. There has been an unmet need, particularly in the economy automotive market, to reduce the number of layers of paint required to achieve a “show surface without loss of corrosion performance. The coatings of the invention have both higher gloss and smoothness and may be used as a stand alone coating, such as a finish coating. In particular in the economy automotive market, these coatings may be useful for less expensive car body parts thereby providing customers with an attractive, glossy coating at lower cost.